Screw-propeller.



- PATENTED OUT. 8, 1907.

D. w. TAYLOR. Y SCREW PROPELLER. APPLICATION P ILED DEC. 27, 1906.

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m 25,359 550122902; lu ma zu z PATENTED OCT. 8, '1907.

D. W. TAYLOR. SCREW PROPELLER.

APPLICATION FILED 11110.27. 1906.

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WW a 1% DAVID W TAYLOR, OF WASHINGTON, DISTRICT OF COLUMBIA.

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Specification of Letters Patent. Application filed December 27,1906. Serial No. 349,685.

vPatented Oct. 8, 1 907.

To all whom it may concern: 7

Be it known that 1,. DAvrnWA'rsoN TAYLOR, a citi zen of the United States of America, residing at Washington, District of Columbia, United States of America, have invented certain new and useful Improvements in Screw-Propellers; and l do-hereby declare the following to be a full, clear, and exact description of theinventlon, such as will enable others skilledv in the art to which it app'ertains to make and use the same, reference being had to the accompanying drawings, and to letters or figures of reference marked thereon, which form a part-of this specification. I My invention relates to screw propellers and has for its object a construction of blade designed to prevent cavitation and increase the efficiency of such blades especially in propellers driven at high speed.

Referring to the drawings in which like parts as similarly'designated, Figure 1, is a developed section of a propeller blade as most commonly used. 'Fig. 2 is a diagram showing the curves of action of the blades shown in 'Figs. 1 and 3. Fig. 3 is a developed section of my new propeller blade, and Fig. 4 is a longitudinal section of a blade showing three developed sections.

In order to make clear the essential and characteristic features of my new type of propeller blade, I will contrast its construction and method of operation with that of the ordinary ogival type, the most common now in use. Fig. 1 shows a developed section of an ordinary dgival propeller blade. A section of the blade is cut by a cylinder concentric with the shaft and then keeping fast the top line of the cylinder it is unrolled into a plane parallel with the shaft 2'. e. the section is a developed section. The pitch of the driving face beingconstant it develops in Fig, 1 into a straight line ABC. The back of the blade, ADB, as developed, is circular, hence the sectionis a segment of a circle, or

blade'section in Fig. 1 is 2.08 multiplied by thediameter of the section and the thickness of the section in the center, CD, is .083 of the expanded or developed length of face AB. These are common proportions. I

In considering the action of propellers there is often made the approximate assum tion that the water moves only parallel to theshaftaxis, F. E. in Fig. 1. I shall first describe the action upon this assumption. To transform the motion from circular. to plane we assume that the'developed section is moving up and to the right in such a manner that it meets the particles of undisturbed water in its plane at the same angle and in the same way that the actual section of the blade bya cylinder would meet the particles of water in-a cylindrical shell of the radius of the section. If there were no slip, thesection in, Fig. 1 would advance parallel to its facfAC. For a slip of 20 per cent calculated on the face pitch the section advances in the direction CK. Drawing parallel to OK it is seen that the reaction, or thrust.

' less'than the pitch of the face.

different. enormous negative value of 128%, retains a'negative leading edge A advancing parallel to LA splits the water so that a particle as G, which is slightly to the rear of the line LA, is forced sternward, giving a useful But a particle, H, adjacent to G but just forward of the line LA, is forced forward and its reaction is prejudicial to the action of the screw, since it results in a sternward thrust on the propeller, while the only useful thrust is the forward thrust. It is evident that while the face of the blade may have a uni-' form ,pitch the back of the blade has a different pitch at each point. At D the back is parallel to the face,

andhence the pitch is the same as that of the face. As we pass from D to A the back assumes a greater and greater angle with CA, so that the pitch. of the back steadily falls off as we passfrom D to A, and in the vicinity of A the pitch of the leading edge is very much Conversely, as we pass from D to B, the pitch of the back steadily increases, and in the vicinity of B, the following edge, is very much greater than that of the face. i

'I have calculated for the section in Fig. 1 the effect of this variation of pitch upon the slip, i. e. upon the face of the blade, face or back. The results are shown in Fig. 2. It is seen that for 20 per cent slip as usally calculated, the slip along the face of the ordinary section jumps at once to 20% at the leading edge and remains constant to the following edge. Along the back of the ordinary section, however, the case is very The slip starts at the leading edge with the value for nearly the length of the blade and then continues to increase until at the following edge the p slip along the back of the blade is over as against of the ogival type. The pitch of the face AGB, of the 20% for the face. The result, under the assumed conditions of action, is that a particle of water' which isjust caught by the driving face of the blade is suddenly accelerated sternward until its velocity is 20% of the speed of advance of the screw' (speed of advance= pitchXrevolutions). An adjacent particle which is just caught by the back of the blade is given a sudden acceleration forward until its velocity forward is actually more than 1% times the speed of advance of the screw. Then, as it passes to the following edge, it is continually and strongly accelerated sternward until at the following edge its actual sternward velocity is over 60% of the speed of advanced the screw.

The conditions shown in Fig. 2, as already stated, are deduced from the assumption that the particles of water move always parallel to the axis of the screw and that the blade section advances into water at rest. These assumptions are not exact, and it will be well to considcrthe departures from them in actual practice.

In the place propellers in action affect, through motion of a particle of water in contact with the face suction, t l 1 e water for some distance forward of them. The pressure of this water is reduced and the water is given a sternward velocity so that in practice the leading edge, A, of the section does not advance into undisturbed water but into water which already has a certain sternward velocity. The effect of this is that the acceleration of the particle G, which is caught bythe driving face, is not so sudden as would be indicated by Fig. 2. The 20% slip is not reached instantly but through a more or less gradual process. When we consider, however, the action upon .11, or the particle of water which is just caught by the back of the blade, we find the conditions worse than indicated in Fig. 2. This particle, instead of being accelerated forward from a condition of rest, has, before it reaches the blade a velocity sternward. This .is rapidly changed to a forward velocity, and then the velocity-varies as indicated in Fig. 2. Such sudden and abrupt reversals of velocity and acceleration are prejudicial to efficient action. In the second place, the water as it passes through the propeller disk is necessarily given some velocity of rotation as well as translation aft. The'rotation, however, is practically all communicated after the water has reached the screw disk, the effect of the suction of the propeller upon the water forw'ard being simply to give it welocity aft and some velocity inward, the waterconverging from-forward towards the propeller. Since the velocity of rotation is communicated after the water reaches'the propeller disk and is zero at the leading edge, the effect of this velocity upon the action of the .forward portionof the blade is comparatively slight. The effect upon the rear portion would be to reduce the fore and aft slip so that the .curves of slip in Fig. 2 somewhat exaggerate the real slip, especially as regardsthe back of the blade. Finally, owing to the thickness of the blade, the water through which it passes is given velocityto enable the blade to get through. Broadly speaking, this effect is that from almost at the leading edge, A, the water on each side of the section is fiowingback parallel to the section towards the following edge. The velocity of flow is at' a maximum abreastCD, the thickest part of the blade, and then thisvelocity graduallyfalls off until we reach the immediate vicinity of B, the following edge. The effect of this internal motion of the water would be to slightly modify the curves of Fig. 2. in a somewhat indeterminate way, but not very seriously. We are justified then in concluding that the slip curves of Fig. 2 represent with reasonable approximation the conditions of action of a section of a propeller blade,

the propeller necessarily involves rapid alterations of velocity and acceleration in the vicinity of the .backof the blade. Now to produce acceleration and velocity sternward the back of the blade must rely upon suction.

For moderate speeds the forces necessary to produce acceleration are not very-large, the water willremain throughout in contact with the back of the blade, and the final thrust or reaction upon the blade will depend almost entirely upon the average rearward velocity with which the water leaves B, the following edge. Hence for moderate speeds and pressures'the ultimate result ofthese violent fluctuations of acceleration and velocity is not very pronounced and consists entirely in a reduction of the efficiency with which the screw acts, a reduction which in many cases is probably not large and incapable of detection under practical con ditions of operation. When, however, we pass to very high speeds and high pressures we reach a different; condition. The water, in order to follow the back of the-blade, must experience very large changes of velocity, acceleration and pressure. We finally reach a condition where the water, having nothing butthe suction of the back of the blade to impel it, is unable to respond so as to follow the back of the blade. The blade, in homely parlance, loses its grip, and we have the phenomenon called cavitation, which has been frequently observed with screws developing heavy thrusts and running at high speeds. As manifested in a screw attached to a ship, cavitation makes itself known by a rather sudden and abnormal increase of slip'and a corresponding increase in the expenditure of power without any correspondingincrease in thrust. In order to avoid cavitation we must avoid its cause, namely, the sudden variations of acceleration and velocity of the water passing through the screw disk in the vicinity of the backs of the blades. If a blade could be made without thickness the .problem would be comparatively easy. We know that a particle moving alpng a parabola with a uniform yelocity in a direction perpendicular to the axis has auniiorm acceleration parallel to the axis of the parabola. If then we made a blade which had no thickness a parabola in section, such a blade would have uniform pitch radially and varying pitch axially and a particle of water passing through the screw disk, either along the front or the back of the blade, would be given uniform acceleration on its way through from the leading edge to the follow ingedge. It would be necessary then only to arrange the pitch of the leading edge so that it would act without shock upon the water which it meets, and which will already have some sternward velocity owing to the suction of the propeller.

4 In Fig. 3 B OP is a curved directrix, specifically a parabola which has at O the same pitches the driving face ACB of the propeller section in v1. Its pitch at P is 20% less and its pitch at'B is 20% greater. If

. this section could be an actual blade it would undoubtsimilar smooth curve, but this curve has been utilised for the driving face. An examination of Figs. 1 and 2 will show that the effect of curving the face AGB would be to still further intensify the features ofactionof the ordinary section which are so conducive to cavitation. The pitch of the leading portion of the back of the blade would be still further decreased, and the negative slip in action of this portion of theblade still further increased.

It is necessary for propeller bladesto have thickness, and the problem is to arrange the thickness in the manner least conducive to cavitation without interioring with elliciency. This I accomplish as follows: Taking the parabolic curve POB or any similar smooth curve, (being a developed section of an ideal blade without thickness, having increasing pitch axially and constant pitch radially) as directrix, and at its center, 0, set off C D either symmetrically or ,seasss proportionally the maximum thickness of the blade section This thickness isfixed by considerations of strength. From D draw D A parallel to the directing 'curve or, Set Orr from 13 A, the thickness of the blade set'ofi -I establish the arc A for the forward half of the having |vio1ent oscillations, and the result is that this at various points and drawing a line through the points driving face. For the rear portion ofjthesection', using OB as directrix, set off the thickness either proportionally or. equally on each side ofiOB as indicated in the same, manner as (1 and D have been set off. The curve of thickness may be ogival or parabolic, but I prefer to use a curve 'of since as giving smaller angles at the entering andleaving-edges, a feature conducive to efliciency.

In Fig. 3, as already. stated, the directrix B OP is a parabola and Fig. 2 full lines shows the curves of 'slip along the face and back of the new section comparable to the"cni'ves forthe ordinary section Fig. 1. These curves are fora nominal slip of 20% at the center, the same as in the dotted curves. It is seen that there is no negative slip at any point. The slip along the face,

I instead of starting at the leadingedge with a value of high slip of 70 on the drivingface atthe loading edge is objectionable, it is more or less unavoidable if theblade is to have thickness, and it is to be noted that it is positive, that is to say the water is positively forced astern with this high slip and not coaxed by suction.

With the new section the acceleration of the water along the back of the blade is nearly uniform instead of section will-work without cavitation toa very inuch higher velocity, or'wliat isof equal practical value,

blades of this type of section may be made much nar- 'rower than blades of the ordinary type without cavitation With the ordinary type of section cavitation may frequently be avoided by making the blades very broad. This decreases the angles of entering and leading edges, reduces the' negative slip and hence the oscillations in the sternward acceleration and gives a longer time for the water to remain within the screw disk and.

' begiven the necessary accelerations. But the effect of the very broad blades with quick running screws is to very much increase the loss through friction of blade surface, without any corresponding increase in thrust,

and hence such screws are unavoidably of low efficicncy. By adopting my typefof section much narrower blades may be used, which are equally as good as regards cavitation and are much more efficient than the broad: blades. The governing feature is the curved .dircctrix utilized as already described. I Y

Fig. 2 shows the approximate slip in a fore and aft direction over the whole section of Fig"3 for a nominal slip, or slip at center of blade of 20%. Under these conditions the section advances parallel to A M,, Fig. 3, and this line is tangent to the back of the blade at iisince the pitch here is that of OP at P, and this was made 20% less than that of the directrix at 0. To

avoid. cavitation, howeventhis section must, and is intended to, work at a greater slip than 2 0% the line of advance being some liue-A N corresponding, say, to

. 30 slipand making an'anglewith A M the tangent to be back ofthe blade. This is because the water at A has already some sternward velocity when men-and to avoid any forward acceleration the back of the blade at A must move at an angle to itstangen't. This construction is not favorable to efliciency for propellers designed to operate under conditions where cavitation is not seriously to be apprehended for the reason that to operate at very large slip with some loss of efficiency.'. For such propellers the line of advance, instead of being as A N in Fig. 3, should, fall slightly on the other side of A,M,', as A,N This construction enables the blade section to enter undisturbed water in the most efficient fashion, and is preferable when the conditions of operation are such that the water will remain in contact with the blade during operation in spite of the slight forward velocity given it at first along the leading portion of the back of the blade section.

From consideration of the analysis of propeller ac-f tion I have given, it might'seem at first that the back of the blade should parallel the directrix all the way instead of for the forward half only. This construction,

however, is seriously objectionable in practice." The angles involved are small and the parabolic or curved directrix is not greatly curved, although a small curvature is essential to smooth action. iIn fact, it is not .it requires the'leading portion of the face of the blade generally recognized or known how materially the ac I tion of propellers is affected by comparatively small variations of angle. Since, then, the directrix is not greatly curved, if it were used for the whole back of the blade the face of the blade would be very rounding,

being not very unlike the ogival section reversed, and r the result would be that under conditions where there is risk of cavitation the water would find it difficult to follow the rear portion of the face, and there would be cavitation due to the face action instead of the back.

' disk, there would be decrease of rearward acceleration of the water along the following portion of the back of the blade if it were parallel to the parabolic directrix,

- and the departure from this shape does not result in increasing sternward acceleration to the extent that .This-result would not follow if the blades could be made very thin, but in practice propeller blades must would be inferred from consideringthe water (as in I Fig. 2 full lines) restricted. to fore and aft motion only.

' In Fig. 4 the pitchof the leadingedge of each directrix' is'substantially one half of the diameter of the screw propeller and the pitch of the following edge of each directrix-is substantially 1.25 times the diameter of the screw propeller.

1. A propeller blade whose developed section is derived from a curved directrix extending from the forward or leading part of the blade to the following edge, the blade having the forward half of its back parallel to said directrix, the rear halves of the back and face of the blade converging to meet the directrix at the following'edge and the forward half of the face of the blade determined from the thickness of the section and the curved back of the blade. 4 I

2. A propeller blade \vhose developed section is derived from a parabolic directrix extending from front to rear of the blade, the blade having the forward halfof its back parallelto the directrlx and the rear halves of the sections are derived by proportionally distributing the thickness on the opposite sides of a curved directing surface of axially increasing pitch and constant radial pitch.

5. A propeller blade the back of thelorward half of whose developed section is parallel to a curved dircctrix passing through the following' edge and the face of whose forward-halt is curved from the center of the blade to meet the back at the leading edge.

6. A propeller blade of which the thickness of any section thereof, at any point of the length of said section, is proportional to the ordinates of a curve of sincs that is plotted upon the length of said section which is taken to represent 180.

7. A propeller blade the thickness of any section of which at any point of its rear hall is pr0portion:llly distributed onboth sides of a curved directrix, the whole thickness of the blade section at such point being proportional to the ordinates ot' a curve of sines plotted upon the length of the entire section, which length is taken to represent 180.

In testimony that I claim the foregoing as my invention, I have signed my name in presence of two subscribins witnesses.

D. W. TAYLOR.

Witnesses:

lnlnir I Lanxnn, llnNnr Olrru, J r. 

